Planar junction diodes having a small area contact can emit very high brightness and are therefore valuable sources for fiber optic communications at bit rates below about 100 mHz. However, they suffer from the disadvantage that if the emission takes place perpendicular to the junction the light output increases slower than linearly with drive current, whereas if the emission takes place parallel to the junction plane the light output increases faster than linearly with drive current so that in either instance the device is non-linear. This is a serious drawback for various applications, such as multiplex analogue data transmission. The cause of this non-linearity is fundamental and arises because the high carrier injection levels used create sufficient population inversion for stimulated emission to occur giving gain and therefore super-radiance for propagation in the junction plane. This super-radiance results in the light output-current input characteristic being super-linear for the component propagating in the junction plane and sub-linear for the component propagating perpendicular to the junction plane. By combining these components together in a suitable manner it is possible to achieve a near linear output characteristic. Combination can be achieved either externally using optical components, as described in our co-pending British Patent Application No. 18078/74 (M. M.Ramsay-12), or internally within the diode chip itself by using an appropriate geometry.